Hi all in GadgetmanLand!
Well, with some recent findings on the effect of evap & emissions systems on an engines vac system, I thought I'd find out how it works - from a GM platform. Other manufactures will probably be similar.
The evaporative emission(EVAP) controls system limits fuel vapors from escaping in to the atmosphere. Fuel vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs & stores the fuel vapors. Excess pressure is vented through the vent line & EVAP canister vent solenoid valve to atmosphere. The EVAP canister stores fuel vapors untill the engine is able to use them. At the appropriate time, the control module will command the EVAP canister purge solenoid valve ON, or "open", allowing engie vacuum to be applied to the EVAP canister. With the EVAP canister vent solenoid valve off, or "open", fresh aire will be drawn through the solenoid valve and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP canister purge solenoid valve in the intake manifold.
The above "short course" is obviously canned, but gives you some idea of the process. This process will happen as you are driving down the road, determined by the ECM. If the system does not see proper vac when it's supposed to, codes will set. Likewise, when pressure is checked and found to be lacking, codes will set. Usually the Purge solenoid or Vent solenoid valve are at fault. But on older, rusty cars, rusty pipes and fuel fill necks can also be at fault if they leak vapors.
Anyways, with the developments found on Micheal's 5.0 liter Explorer, I thought I'd ck on this. Hope this helps. It was a good refresher course for myself.
Keep Grooveing!
Scott Castleberg
Pellston, MI
Gadgetman Michigan
